Bimodal polyethylene for injection stretch blow moulding applications

ABSTRACT

A polyethylene resin having a multimodal molecular weight distribution comprising at least two polyethylene fractions A and B, fraction A being substantially free of comonomer and having a lower weight average molecular weight and a higher density than fraction B, each fraction prepared in different reactors of two reactors connected in series in the presence of a Ziegler-Natta catalyst system, the polyethylene resin having a density of from 0.950 to 0.965 g/cm 3  and a melt index MI2 of from 0.5 to 5 g/10 min.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2010/066703, filed Nov. 3,2010, which claims priority from EP 09175544.7, filed Nov. 10, 2009, andEP 10156984.6, filed Mar. 19, 2010.

FIELD OF THE INVENTION

The present invention relates to a polyethylene resin with a multimodal,preferably bimodal, molecular weight distribution for the preparation ofpolyethylene preforms for one- or two-stageinjection-stretch-blow-moulding (ISBM) processes and to the ISBMarticles produced therefrom.

BACKGROUND OF THE INVENTION

Injection-stretch blow molding (ISBM) is a process widely used for theproduction of containers, such as bottles, using thermoplastic polymers.The process includes the steps of preparing a pre-form by injectionmolding and then expanding the pre-form to the desired final shape. Ingeneral, one distinguishes one-stage and two-stage processes. In theone-stage process the steps of producing the pre-form and expanding thepre-form to the desired final shape are performed in the same machine.In the two-stage process these two steps are performed in differentmachines, in some cases even in different geographical locations; thepre-form is allowed to cool to ambient temperature and is thentransported to a second machine where it is reheated and expanded to thedesired final shape. Due to reasons of production speed and flexibilitythe two-stage process is preferred for larger production volumes.

Recent progress in development has made polypropylene a viablealternative to polyethylene terephthalate (PET) for injection-stretchblow molding (ISBM). Due to their good optical propertiespropylene-ethylene random copolymers are the preferred polypropylenegrades.

For the injection molding of polypropylene it is well known to improvethe impact performance, while also having good optical properties, bythe addition of a polyethylene, which has been produced using ametallocene catalyst.

For example, EP-A-151741 to Mitsui discloses single-stage manufacturingof articles by ISBM. These articles are prepared from propylene-ethylenerandom copolymers having a melt flow index of from 4 to 50 dg/min andcontaining a nucleating agent.

WO95111791 to Bekum is directed to a two-stage process for preparingarticles by ISBM. The preferred resin is an ethylene-propylene copolymercontaining more than 50 wt % of propylene and having a melt index offrom 10 to 20 dg/min.

WO 2005/005143 to Total Petrochemicals discloses blow-molded containersmade from a blend of polypropylene and a metallocene polyethylene toimprove the impact strength.

The polypropylenes presently used in injection-stretch blow moldingapplications allow for the production of containers with good opticalproperties at industrially viable production rates. However, as comparedto other polymers used in injection-stretch blow molding polypropylenesuffers from a lack of the combination of high rigidity and high ESCR,as well as high impact strength, particularly at lower temperatures.

Thus, there is an interest for improving the impact performance,rigidity and ESCR of injection-stretch blow molded containers. A balancehas to be found between the high fluidity required for the first step toform the preform and the lower fluidity required for the second stepwhen blowing the preform.

JP2000086722 to Asahi discloses the use of high-density polyethylene,preferably prepared with a metallocene catalyst, suitable for injectionstretch blow molding.

JP2000086833 to Asahi discloses the use of resin compositions suitablefor injection stretch blow molding at a high stretch ratio, comprising apolyethylene prepared with a metallocene catalyst and a polyethyleneprepared with a chromium catalyst.

JP9194534 to Mitsui discloses the use of a polyethylene-based resin forinjection stretch blow molding having a density of 0.940 to 0.968 g/cm³and a melt flow index of 0.3 to 10 g/10 min (ASTM D1238 at 190° C. and2.16 kg).

It is an aim of the invention to provide a polyethylene resin forinjection stretch blow moulding with a broad processing window.

It is also an aim of the invention to provide a polyethylene resin forinjection stretch blow moulding with good process stability.

It is an aim of the invention to provide a polyethylene resin forinjection stretch blow moulding with a high environmental stress crackresistance (ESCR measured with 100% Igepal CO-630). The environmentalstress crack resistance is advantageously of at least 100 h, preferablyat least 400 h.

In addition is an aim of the invention to provide a polyethylene resinfor injection stretch blow moulding with a high impact resistance.

Furthermore, it is an aim of the invention to provide a polyethyleneresin for injection stretch blow moulding with high rigidity.

In addition, it is also an aim of the invention to provide apolyethylene resin for injection stretch blow moulding to preparecontainers with a high top load. The top load is the ability of astanding bottle to withstand the weight of other bottles on pallets.

It is further an aim of the invention to provide a polyethylene resinfor injection stretch blow moulding to prepare containers with goodthickness repartition.

It is additionally an aim of the invention to provide a polyethyleneresin for injection stretch blow moulding to prepare containers withgood surface aspects.

It is furthermore an aim of the invention to provide a polyethyleneresin for injection stretch blow moulding to prepare containers withgood finishing for molded drawings.

Finally, it is also an aim of the invention to provide a polyethyleneresin suitable for injection stretch blow moulded containers forconsumer packaging, in particular for cosmetics and detergents.

At least one of these aims is fulfilled by the resin of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the molecular weight distribution of two polyethyleneresins having a bimodal molecular weight distribution, Grade Y being aresin according to the invention. The bimodality is shown as a shoulderon the gaussian curve at around Log(M)=4.8

FIG. 2 shows a bottle obtained by ISBM with Grade Z according to theexamples

FIG. 3 shows a preform with random flow lines (made with Grade Xaccording to the examples)

FIG. 4 shows the side view of an ISBM bottle made with Grade X accordingto a comparative example

FIG. 5 shows the bottom view of a an ISBM bottle made with Grade Yaccording to the invention

FIG. 6 shows the schematics of a bottle design

FIG. 7 shows an ISBM Bottle made with Grade Y according to the invention

SUMMARY OF THE INVENTION

A polyethylene resin having a multimodal molecular weight distributioncomprising at least two polyethylene fractions A and B, fraction A beingsubstantially free of comonomer and having a lower weight averagemolecular weight than fraction B and a higher density than fraction B,each fraction prepared in different reactors of two reactors connectedin series in the presence of a Ziegler-Natta catalyst system, thepolyethylene resin having a density of from 0.950 to 0.965 g/cm³,measured following the method of standard test ASTM 1505 at atemperature of 23° C., a melt index MI2 of from 0.5 to 5 g/10 min,measured following the method of standard test ASTM D 1238 at atemperature of 190° C. and under a load of 2.16 kg, and molecular weightdistribution Mw/Mn of from 5 to 20.

By “substantially free of comonomer” it is meant that the polymerisationstep to obtain the polyethylene fraction A is carried out in the absenceof comonomer.

The resin according to the invention is particularly suitable forinjection stretch blow moulding (ISBM). Thus the invention also coversinjection stretch blow moulded articles, in particular containers,preferably containers for consumer packaging e.g. for cosmetics ordetergents, as well as the use of the resin according to the inventionfor ISBM applications.

The process for obtaining the resin is also included herein.

DETAILED DESCRIPTION OF THE INVENTION The Polyethylene Resin

The polyethylene resin having a multimodal, preferably bimodal,molecular weight distribution according to the present invention can beproduced by polymerizing ethylene and one or more optional comonomers inthe presence of a Ziegler-Natta catalyst system in two or more reactorsconnected in series. Due to the use of two or more reactors, the resinaccording to the invention comprises a high molecular weight (HMW), lowdensity fraction and a low molecular weight (LMW), high densityfraction.

Any Ziegler-Natta system known to the person skilled in the art can beused. A preferred Ziegler-Natta catalyst system comprises a titaniumcompound having at least one titanium-halogen bond and an internalelectron donor, both on a suitable support (for example on a magnesiumhalide in active form), an organoaluminium compound (such as analuminium trialkyl), and an optional external donor.

More preferably, the Ziegler-Natta catalyst system used to prepare thepolyethylene resin of the present invention comprises a Ziegler-Nattacatalyst component D and a preactivating agent, wherein the ZieglerNatta catalyst component D is obtainable by:

-   -   a) generating a reaction product A by contacting a magnesium        dialkoxide compound with a halogenating agent;    -   b) contacting reaction product A with a first        halogenating/titanating agent to form reaction product B;    -   c) contacting reaction product B with a second        halogenating/titanating agent to form reaction product C; and    -   d) contacting reaction product C with a third        halogenating/titanating agent to form catalyst component D.

Products A and B are not be confused with the polyethylene fractions Aand B of the resin.

Preferably, the preactivating agent is an organoaluminium compound,preferably of the formula AlR₃, wherein R is an alkyl having 1-8 carbonatoms or a halide, and wherein each R may be the same or different. Morepreferably, the organoaluminium compound is TEAL.

Preferably, the halogenating agent is ClTi(OPr)₃.

Preferably, the first halogenating/titanating agent a mixture of TiCl₄and Ti(OBu)₄, in a molar ratio range of from 0.5:1 to 6:1 ofTiCl₄/Ti(OBu)₄. More preferably the molar ratio is 2:1 ofTiCl₄/Ti(OBu)₄.

Preferably, the second halogenating/titanating agent is TiCl₄.

Preferably, the third halogenating/titanating agent is also TiCl₄.

By “Ziegler-Natta catalyst system,” we mean a Ziegler-Natta catalystcomponent in combination with a preactivating agent.

By “Ziegler-Natta catalyst component,” we mean a transition metalcompound that incorporates a Group 4-8 transition metal, preferably aGroup 4-6 transition metal, and one or more ligands that satisfy thevalence of the metal. The ligands are preferably halide, alkoxy,hydroxy, oxo, alkyl, and combinations thereof. Ziegler-Natta catalystsexclude metallocenes or other single-site catalysts.

It is thought that the Ziegler-Natta catalyst used in the process ofthis invention without being bound to theory has the effect that theresin has an overall higher molecular weight (i.e. higher extrudateswell) without affecting the low molecular weight tailing (i.e. betterimpact properties).

The present invention provides a polymerisation process wherein thecatalyst is preferably made according to a process comprising thefollowing steps:

a) contacting a magnesium dialkoxide compound with a halogenating agentto form a reaction product A;

b) contacting reaction product A with a first halogenating/titanatingagent to form reaction product B;

c) contacting reaction product B with a second halogenating/titanatingagent to form reaction product C;

and d) contacting reaction product C with a thirdhalogenating/titanating agent to form reaction product D.

The second and third halogenating/titanating agents can comprisetitanium tetrachloride. The second and third halogenating/titanatingsteps can each comprise a titanium to magnesium ratio in the range ofabout 0.1 to 5. The reaction products A, B and C can each be washed witha hydrocarbon solvent prior to subsequent halogenating/titanating steps.The reaction product D can be washed with a hydrocarbon solvent untiltitanium species [Ti] content is less than about 100 mmol/L.

Another embodiment of the present invention provides a polyolefincatalyst produced by a process generally comprising contacting acatalyst component of the invention together with an organometallicagent. The catalyst component is produced by a process as describedabove. The catalysts of the invention can have a fluff morphologyamenable to polymerization production processes, and may provide apolyethylene having a molecular weight distribution of at least 5.0 andmay provide uniform particle size distributions with low levels ofparticles of less than about 125 microns. The activity of the catalystis dependent upon the polymerization conditions. Generally the catalystwill have an activity of at least 5,000 gPE/g catalyst, but the activitycan also be greater than 50,000 gPE/g catalyst or greater than 100,000gPE/g catalyst.

Even another embodiment of the present invention provides a polyolefinpolymer produced by a process comprising: a) contacting one or moreolefin monomers together in the presence of a catalyst of the invention,under polymerization conditions; and b) extracting polyolefin polymer.Generally the monomers are ethylene monomers and the polymer ispolyethylene.

According to one embodiment of the invention, a method for making acatalyst component generally includes the steps of forming a metaldialkoxide from a metal dialkyl and an alcohol, halogenating the metaldialkoxide to form a reaction product, contacting the reaction productwith one or more halogenating/titanating agent in three or more steps toform a catalyst component, and then treating the catalyst component witha preactivation agent such as an organoaluminum.

One embodiment of the present invention can be generally as follows:

-   -   1. MRR′+2R″OH→M(OR″)₂    -   2. M(OR″)₂+ClAR′″_(x→)“A”    -   3. “A”+TiCl₄/Ti(OR″′)_(4→)“B”    -   4. “B”+TiCl_(4→)“C”;    -   5. “C”+TiCl_(4→)“D”    -   6. “D”+preactivating agent→catalyst

In the above formulas, M can be any suitable metal, usually a Group IIAmetal, typically Mg. In the above formulas, R, R′, R′, R″′, and R″″ areeach independently hydrocarbyl or substituted hydrocarbyl moieties, withR and R′ having from 1 to 20 carbon atoms, generally from 1 to 10 carbonatoms, typically from 2 to 6 carbon atoms, and can have from 2 to 4carbon atoms. R″ generally comprises from 3 to 20 carbon atoms, R′″generally comprises from 2-6 carbon atoms, and R″′ generally comprisesfrom 2-6 carbon atoms and is typically butyl. Any combination of two ormore of R, R′, R″, and R″″ can be used, may be the same, or thecombination of the R groups may be different from one another.

In the above embodiment comprising formula ClAR′″_(x), A is anon-reducing oxyphilic compound which is capable of exchanging onechloride for an alkoxide, R″′ is a hydrocarbyl or substitutedhydrocarbyl, and x is the valence of A minus 1. Examples of A includetitanium, silicon, aluminum, carbon, tin and germanium, typically istitanium or silicon wherein x is 3. Examples of R′″ include methyl,ethyl, propyl, isopropyl and the like having 2-6 carbon atoms.Nonlimiting examples of a chlorinating agent that can be used in thepresent invention are ClTi(O^(i)Pr)₃ and ClSi(Me)₃.

The metal dialkoxide of the above embodiment is chlorinated to form areaction product “A”. While the exact composition of product “A” isunknown, it is believed that it contains a partially chlorinated metalcompound, one example of which may be ClMg(OR″).

Reaction product “A” is then contacted with one or morehalogenating/titanating agent, such as for example a combination ofTiCl₄ and Ti(OBu)₄, to form reaction product “B”. Reaction product “B”which is probably a complex of chlorinated and partially chlorinatedmetal and titanium compounds. Reaction product “B” can comprise atitanium impregnated MgCl₂ support and for example, may possibly berepresented by a compound such as (MCl₂)_(y) (TiCl_(x)(OR)_(4-x))_(z).Reaction product “B” can be precipitated as a solid from the catalystslurry.

The second halogenation/titanation step produces reaction product, orcatalyst component, “C” which is also probably a complex of halogenatedand partially halogenated metal and titanium compounds but differentfrom “B” and may possibly be represented by (MCl₂)_(y)(TiCl_(x′)(OR)_(4-x′))_(z′). It is expected that the level ofhalogenation of “C” would be greater than that of product “B”. Thisgreater level of halogenation can produce a different complex ofcompounds.

The third halogenation/titanation step produces a reaction product, orcatalyst component, “D” which is also probably a complex of halogenatedand partially halogenated metal and titanium compounds but differentfrom “B” and “C”, and may possibly be represented by(MCl₂)_(y)(TiCl_(x″)(OR)_(4-x″))_(z″). It is expected that the level ofhalogenation of “D” would be greater than that of product “C”. Thisgreater level of halogenation would produce a different complex ofcompounds. While this description of the reaction products offers themost probable explanation of the chemistry at this time, the inventionas described in the claims is not limited by this theoretical mechanism.

Metal dialkyls and the resultant metal dialkoxides suitable for use inthe present invention can include any that can be utilized in thepresent invention to yield a suitable polyolefin catalyst. These metaldialkoxides and dialkyls can include Group IIA metal dialkoxides anddialkyls. The metal dialkoxide or dialkyl can be a magnesium dialkoxideor dialkyl. Non-limiting examples of suitable magnesium dialkyls includediethyl magnesium, dipropyl magnesium, dibutyl magnesium,butylethylmagnesium, etc. Butylethylmagnesium (BEM) is one suitablemagnesium dialkyl.

In the practice of the present invention, the metal dialkoxide can be amagnesium compound of the general formula Mg(OR″)₂ where R″ is ahydrocarbyl or substituted hydrocarbyl of 1 to 20 carbon atoms.

The metal dialkoxide can be soluble and is typically non-reducing. Anon-reducing compound has the advantage of forming MgCl₂ instead ofinsoluble species that can be formed by the reduction of compounds suchas MgRR′, which can result in the formation of catalysts having a broadparticle size distribution. In addition, Mg(OR″)₂, which is lessreactive than MgRR′, when used in a reaction involving chlorination witha mild chlorinating agent, followed by subsequenthalogenation/titanation steps, can result in a more uniform product,e.g., better catalyst particle size control and distribution.

Non-limiting examples of species of metal dialkoxides which can be usedinclude magnesium butoxide, magnesium pentoxide, magnesium hexoxide,magnesium di(2-ethylhexoxide), and any alkoxide suitable for making thesystem soluble.

As a non-limiting example, magnesium dialkoxide, such as magnesiumdi(2-ethylhexoxide), may be produced by reacting an alkyl magnesiumcompound (MgRR′) with an alcohol (ROH), as shown below.MgRR′+2R″OH→Mg(OR″)₂+RH+R′H

The reaction can take place at room temperature and the reactants form asolution. R and R′ may each be any alkyl group of 1-10 carbon atoms, andmay be the same or different. Suitable MgRR′ compounds include, forexample, diethyl magnesium, dipropyl magnesium, dibutyl magnesium andbutyl ethyl magnesium. The MgRR′ compound can be BEM, wherein RH and R′Hare butane and ethane, respectively.

In the practice of the present invention, any alcohol yielding thedesired metal dialkoxide may be utilized. Generally, the alcoholutilized may be any alcohol of the general formula R″OH where R″ is analkyl group of 2-20 carbon atoms, the carbon atoms can be at least 3, atleast 4, at least 5, or at least 6 carbon atoms. Non-limiting examplesof suitable alcohols include ethanol, propanol, isopropanol, butanol,isobutanol, 2-methyl-pentanol, 2-ethylhexanol, etc. While it is believedthat almost any alcohol may be utilized, linear or branched, a higherorder branched alcohol, for example, 2-ethyl-1-hexanol, can be utilized.

The amount of alcohol added can vary, such as within a non-exclusiverange of 0 to 10 equivalents, is generally in the range of about 0.5equivalents to about 6 equivalents (equivalents are relative to themagnesium or metal compound throughout), and can be in the range ofabout 1 to about 3 equivalents.

Alkyl metal compounds can result in a high molecular weight species thatis very viscous in solution. This high viscosity may be reduced byadding to the reaction an aluminum alkyl such as, for example,triethylaluminum (TEAl), which can disrupt the association between theindividual alkyl metal molecules. The typical ratio of alkyl aluminum tometal can range from 0.001:1 to 1:1, can be 0.01 to 0.5:1 and also canrange from 0.03:1 to 0.2:1. In addition, an electron donor such as anether, for example, diisoamyl ether (DIAE), may be used to furtherreduce the viscosity of the alkyl metal. The typical ratio of electrondonor to metal ranges from 0:1 to 10:1 and can range from 0.1:1 to 1:1.

Agents useful in the step of halogenating the metal alkoxide include anyhalogenating agent which when utilized in the present invention willyield a suitable polyolefin catalyst. The halogenating step can be achlorinating step where the halogenating agent contains a chloride (i.e.is a chlorinating agent).

Halogenating of the metal alkoxide compound is generally conducted in ahydrocarbon solvent under an inert atmosphere. Non-limiting examples ofsuitable solvents include toluene, heptane, hexane, octane and the like.In this halogenating step, the mole ratio of metal alkoxide tohalogenating agent is generally in the range of about 6:1 to about 1:3,can be in the range of about 3:1 to about 1:2, can be in the range ofabout 2:1 to about 1:2, and can also be about 1:1.

The halogenating step is generally carried out at a temperature in therange of about 0° C. to about 100° C. and for a reaction time in therange of about 0.5 to about 24 hours.

The halogenating step can be carried out at a temperature in the rangeof about 20° C. to about 90° C. and for a reaction time in the range ofabout 1 hour to about 4 hours.

Once the halogenating step is carried out and the metal alkoxide ishalogenated, the halide product “A” can be subjected to two or morehalogenating/titanating treatments.

The halogenation/titanation agents utilized can be blends of twotetra-substituted titanium compounds with all four substituents beingthe same and the substituents being a halide or an alkoxide or phenoxidewith 2 to 10 carbon atoms, such as TiCl₄ or Ti(OR″″)₄. Thehalogenation/titanation agent utilized can be a chlorination/titanationagent.

The halogenation/titanation agent may be a single compound or acombination of compounds. The method of the present invention providesan active catalyst after the first halogenation/titanation; however,there are desirably a total of at least three halogenation/titanationsteps.

The first halogenation/titanation agent is typically a mild titanationagent, which can be a blend of a titanium halide and an organictitanate. The first halogenation/titanation agent can be a blend ofTiCl₄ and Ti(OBu)₄ in a range from 0.5:1 to 6:1 TiCl₄/Ti(OBu)₄, theratio can be from 2:1 to 3:1. It is believed that the blend of titaniumhalide and organic titanate react to form a titanium alkoxyhalide,Ti(OR)_(a)X_(b), where OR and X are alkoxide and halide, respectivelyand a+b is the valence of titanium, which is typically 4.

In the alternative, the first halogenation/titanation agent may be asingle compound. Examples of a first halogenation/titanation agent areTi(OC₂H₅)₃Cl, Ti(OC₂H₅)₂Cl₂, Ti(OC₃H₇)₂Cl₂, Ti(OC₃H₇)₃Cl, Ti(OC₄H₉)Cl₃,Ti(OC₆H₁₃)₂Cl₂, Ti(OC₂H₅)₂Br₂, and Ti(OC₁₂H₅)Cl₃.

The first halogenation/titanation step is generally carried out by firstslurrying the halogenation product “A” in a hydrocarbon solvent at roomtemperature/ambient temperature. Nonlimiting examples of suitablehydrocarbons solvent include heptane, hexane, toluene, octane and thelike. The product “A” can be at least partially soluble in thehydrocarbon solvent.

A solid product “B” is precipitated at room temperature following theaddition of the halogenation/titanation agent to the soluble product“A”. The amount of halogenation/titanation agent utilized must besufficient to precipitate a solid product from the solution. In general,the amount of halogenation/titanation agent utilized, based on the ratioof titanium to metal, will generally be in the range of about 0.5 toabout 5, typically in the range of about 1 to about 4, and can be in therange about 1.5 to about 2.5.

The solid product “B” precipitated in this first halogenation/titanationstep is then recovered by any suitable recovery technique, and thenwashed at room/ambient temperature with a solvent, such as hexane.Generally, the solid product “B” is washed until the [Ti] is less thanabout 100 mmol/L. Within the present invention [Ti] represents anytitanium species capable of acting as a second generation Zieglercatalyst, which would comprise titanium species that are not part of thereaction products as described herein. The resulting product “B” is thensubjected to a second and third halogenating/titanating steps to produceproducts “C” and “D”. After each halogenating/titanating step the solidproduct can be washed until the [Ti] is less than a desired amount. Forexample, less than about 100 mmol/L, less than about 50 mmol/L, or lessthan about 10 mmol/L. After the final halogenating/titanating step, theproduct can be washed until the [Ti] is less than a desired amount, forexample, less than about 20 mmol/L, less than about 10 mmol/L, or lessthan about 1.0 mmol/L. It is believed that a lower [Ti] can produceimproved catalyst results by reducing the amount of titanium that canact as a second generation Ziegler species. It is believed that a that alower [Ti] can be a factor in producing improved catalyst results suchas a narrower MWD.

The second halogenation/titanation step is generally carried out byslurrying the solid product recovered from the first titanation step,solid product “B”, in a hydrocarbon solvent. Hydrocarbon solvents listedas suitable for the first halogenation/titanation step may be utilized.The second and third halogenation/titanation steps can utilize adifferent compound or combination of compounds from the firsthalogenation/titanation step. The second and thirdhalogenation/titanation steps can utilize the same agent at aconcentration that is stronger than that used in the firsthalogenation/titanation agent, but this is not a necessity. The secondand third halogenating/titanating agents can be a titanium halide, suchas titanium tetrachloride (TiCl₄). The halogenation/titanation agent isadded to the slurry. The addition can be carried out at ambient/roomtemperature, but can also be carried out at temperatures and pressuresother than ambient.

Generally, the second and third halogenation/titanation agents comprisetitanium tetrachloride. Typically the second and thirdhalogenation/titanation steps each comprise a titanium to magnesiumratio in a range of about 0.1 to 5, a ratio of about 2.0 can also beused, and a ratio of about 1.0 can be used. The thirdhalogenation/titanation step is generally carried out at roomtemperature and in a slurry, but can also be carried out at temperaturesand pressures other than ambient.

The amount of titanium tetrachloride utilized, or alternatehalogenation/titanation agent, may also be expressed in terms ofequivalents, an equivalent herein is amount of titanium relative to themagnesium or metal compound. The amount of titanium of each of thesecond and third halogenating/titanating steps will generally be in therange of about 0.1 to about 5.0 equivalents, can be in the range ofabout 0.25 to about 4 equivalents, typically is in the range of about0.3 to about 3 equivalents, and it can be desirable to be in the rangeof about 0.4 to about 2.0 equivalents. In one particular embodiment, theamount of titanium tetrachloride utilized in each of the second andthird halogenation/titanation steps is in the range of about 0.45 toabout 1.5 equivalent.

The catalyst component “D” made by the above described process may becombined with an organometallic catalyst component (a “preactivatingagent”) to form a preactivated catalyst system suitable for thepolymerization of olefins. Typically, the preactivating agents which areused together with the transition metal containing catalyst component“D” are organometallic compounds such as aluminum alkyls, aluminum alkylhydrides, lithium aluminum alkyls, zinc alkyls, magnesium alkyls and thelike. Preferably, the preactivating agent is selected from the groupconsisting of trialkylaluminums, dialkylaluminum halides, andalkylaluminum dihalides.

The preactivating agent is preferably an organoaluminum compound. Theorganoaluminum preactivating agent is typically an aluminum alkyl of theformula AlR₃ wherein at least one R is an alkyl having 1-8 carbon atomsor a halide, and wherein each of the R may be the same or different.Suitable preactivating agents include trialkyl aluminum such as, forexample, trimethyl aluminum (TMA), triethylaluminum (TEAL),triisobutylaluminum (TIBAL) and also include diethylaluminum chloride,triisobutylaluminum chloride, butylaluminum dichloride, and the like,and mixtures thereof. The organoaluminum preactivating agent is morepreferably trimethyl aluminum (TMA), triethyl aluminum (TEAL),triisobutyl aluminum (TIBAL) or mixtures thereof. Preferably, thepreactivating agent is TEAL, since with TEAL the molecular weightdistribution (MWD) of the bimodal polyethylene prepared in the tworeactors in series is even wider than when using other organoaluminumpreactivating agents. Generally, when using TEAL as the preactivatingagent the MWD will be at least 4.

In general, the ratio of Al to titanium can be in the range from 0.1:1to 2:1 and typically is 0.25:1 to 1.2:1.

Optionally, the Ziegler-Natta catalyst may be pre-polymerized.Generally, a prepolymerization process is affected by contacting a smallamount of monomer with the catalyst after the catalyst has beencontacted with the preactivating agent. A prepolymerization process isdescribed in U.S. Pat. Nos. 5,106,804; 5,153,158; and 5,594,071, herebyincorporated by reference.

Optionally, an electron donor may be added with the halogenation agent,the first halogenation/titanation agent, or the subsequenthalogenation/titanation agent or agents. It may be desirable to have anelectron donor utilized in the second halogenation/titanation step.Electron donors for use in the preparation of polyolefin catalysts arewell known, and any suitable electron donor may be utilized in thepresent invention that will provide a suitable catalyst. Electrondonors, also known as Lewis bases, are organic compounds of oxygen,nitrogen, phosphorous, or sulfur which can donate an electron pair tothe catalyst.

The electron donor may be a monofunctional or polyfunctional compound,can be selected from among the aliphatic or aromatic carboxylic acidsand their alkyl esters, the aliphatic or cyclic ethers, ketones, vinylesters, acryl derivatives, particularly alkyl acrylates or methacrylatesand silanes. An example of a suitable electron donor is di-n-butylphthalate. A generic example of a suitable electron donor is analkylsilylalkoxide of the general formula RSi(OR′)₃, e.g.,methylsilyltriethoxide [MeSi(OEt₃)], where R and R′ are alkyls with 1-5carbon atoms and may be the same or different.

For the polymerization process, an internal electron donor can be usedin the synthesis of the catalyst and an external electron donor orstereoselectivity control agent (SCA) to activate the catalyst atpolymerization. An internal electron donor may be used in the formationreaction of the catalyst during the halogenation orhalogenation/titanation steps. Compounds suitable as internal electrondonors for preparing conventional supported Ziegler-Natta catalystcomponents include ethers, diethers, ketones, lactones, electron donorscompounds with N, P and/or S atoms and specific classes of esters.Particularly suitable are the esters of phthalic acid, such asdiisobutyl, dioctyl, diphenyl and benzylbutylphthalate; esters ofmalonic acid, such as diisobutyl and diethylmalonate; alkyl andarylpivalates; alkyl, cycloalkyl and arylmaleates; alkyl and arylcarbonates such as diisobutyl, ethyl-phenyl and diphenylcarbonate;succinic acid esters, such as mono and diethyl succinate.

External donors which may be utilized in the preparation of a catalystaccording to the present invention include organosilane compounds suchas alkoxysilanes of general formula SiR_(m)(OR′)_(4-m), where R isselected from the group consisting of an alkyl group, a cycloalkylgroup, an aryl group and a vinyl group; R′ is an alkyl group; and m is0-3, wherein R may be identical with R′; when m is 0, 1 or 2, the R′groups may be identical or different; and when m is 2 or 3, the R groupsmay be identical or different.

The external donor of the present invention can be selected from asilane compound of the following formula: wherein R₁ and R₄ are both analkyl or cycloalkyl group containing a primary, secondary or tertiarycarbon atom attached to the silicon, R₁ and R₄ being the same ordifferent; R₂ and R₃ are alkyl or aryl groups. R₁ may be methyl,isopropyl, cyclopentyl, cyclohexyl or t-butyl; R₂ and R₃ may be methyl,ethyl, propyl, or butyl groups and not necessarily the same; and R₄ mayalso methyl, isopropyl, cyclopentyl, cyclohexyl or t-butyl. Specificexternal donors are cyclohexylmethyldimethoxy silane (CMDS),diisopropyldimethoxysilane (DIDS) cyclohexylisopropyl dimethoxysilane(CIDS), dicyclopentyldimethoxysilane (CPDS) or di-t-butyldimethoxysilane (DTDS).

According to the present invention the polyethylene resin is prepared intwo or more serially connected reactors, preferably loop reactors, morepreferably slurry loop reactors, most preferably liquid full loopreactors in the presence of same or different Ziegler-Natta catalystsystems.

Preferably, the high density and low density fractions are produced intwo serially connected loop reactors with the same catalyst system.While preferably the HMW polyethylene fraction is produced in the firstreactor and the LMW polyethylene fraction is produced in the secondreactor, the opposite order is also possible. That is, the lowermolecular weight polyethylene can also be produced in the first of thetwo reactors connected in series. The M_(w) in each of the zones can beregulated by known techniques such as choice of catalyst, reactortemperature, and amount of hydrogen used.

The catalyst system may be employed in a solution polymerisationprocess, a slurry polymerisation process or a gas phase polymerisationprocess. Preferably a slurry process is used. The most preferredpolymerisation process is carried out in two serially connected slurryloop reactors, advantageously liquid full loop reactors i.e. a doubleloop reactor.

In a preferred arrangement, the product of a first reactor, includingthe olefin monomer, is contacted with the second co-reactant and thecatalyst system in a second reactor to produce and mix the secondpolyolefin with the first polyolefin in the second reactor. This is alsoknown as a chemical blend. The first and second reactors areconveniently interconnected, i.e. serially connected, reactors such asinterconnected loop reactors. It is also possible to introduce into thesecond reactor fresh olefin monomer as well as the product of the firstreactor.

Because the second polyolefin is produced in the presence of the firstpolyolefin a multimodal or at least bimodal molecular weightdistribution is obtained.

In one embodiment of the invention, the first co-reactant in the firstreactor is hydrogen, to produce the LMW fraction and the secondco-reactant in the second reactor is the comonomer to produce the HMWfraction. Typical comonomers include hexene, butene, octene ormethylpentene, preferably hexene.

In an alternative embodiment, the first co-reactant in the first reactoris the comonomer, preferably hexene. Homopolymerisation then takes placein the second reactor with little or no interference from the comonomer.Preferably, unreacted comonomer is removed before the polyethylenefraction from the first reactor is transferred to the second reactor.

The temperature in each reactor may be in the range of from 60° C. to110° C., preferably from 78° C. to 98° C.

The high molecular weight, low density fraction has a density of atleast 0.908 g/cm³, preferably of at least 0.922 g/cm³ and of at most0.938 g/cm³, more preferably of at most 0.945 g/cm³. Most preferably itis of about 0.936 g/cm³. It has a high load melt index HL275 of at least1.5 dg/min, more preferably of at least 5 dg/min and most preferably ofat least 7 dg/min and of at most 14 dg/min, more preferably of at most10 dg/min. Most preferably, it is of 8 to 9 dg/min. The HLMI can becalculated from the HL275 by:HLMI=HL275/3.2

The low molecular weight, high density fraction has a density of atleast 0.953 g/cm³, more preferably of at least 0.957 g/cm³, and of atmost 0.978 g/cm³, more preferably of at most 0.962 g/cm³. Mostpreferably it is of about 0.957 to 0.976 g/cm³.

The HLMI and density of the fraction in the second reactor weredetermined using the following formula:Log HLMI_(final)=wt %_(1st)×Log HLMI_(1st)+wt %_(2nd)×Log HLMI_(2nd)density_(final)=wt %_(1st)×density_(1st)+wt %_(2nd)×density_(2nd)wherein

-   -   “final” means “of the polyethylene resin”    -   “1st” means “of the polyethylene fraction produced in the first        reactor”    -   “2nd” means “of the polyethylene fraction produced in the second        reactor, downstream of the first reactor”

The final resin according to the invention has a density of from 0.950to 0.965 g/cm³, preferably 0.952 to 0.962 g/cm³, more preferably 0.954to 0.962 g/cm³ and most preferably 0.957 to 0.960 g/cm³. Thepolyethylene resin has a melt index MI2 of from 0.5 to 5 g/10 min,preferably 0.8 to 3 g/10 min.

Density is measured according to ASTM 1505 at a temperature of 23° C.

HL275 is measured according to ASTM D 1238 at a temperature of 190° C.and under a load of 21.6 kg, except that a die of 2.75 mm broad insteadof 2.1 mm was used.HLMI=HL275/3.2

The melt index MI2 and high load melt index HLMI are measured by themethod of standard test ASTM D 1238 respectively under a load of 2.16 kgand 21.6 kg and at a temperature of 190° C.

The molecular weight distribution is defined by the ratio Mw/Mn of theweight average molecular weight Mw to the number average molecularweight Mn as determined by gel permeation chromatography (GPC).

Preferably the polyethylene resin comprises 36 to 50 wt % of HMWfraction, preferably from 38 to 46 wt %, more preferably from 40 to 43wt % and from 50 to 64 wt % of LMW fraction, preferably from 54 to 62 wt% and most preferably from 57 to 60 wt %. The molecular weightdistribution is preferably of from 5 to 20, more preferably of from 8 to16, most preferably of from 10 to 14. The most preferred polyethyleneresin according to the present invention has a density of about 0.959g/cm³ and a melt index MI2 of about 0.8-1.8 g/10 min and a molecularweight distribution of about 10-14.

The polyethylene resin may contain additives such as, by way of example,antioxidants, light stabilizers, acid scavengers, lubricants, antistaticadditives, nucleating/clarifying agents, and colorants. An overview ofsuch additives may be found in Plastics Additives Handbook, ed. H.Zweifel, 5^(th) edition, 2001, Hanser Publishers.

Injection-Stretch Blow Molding

The polyethylene resin according to the invention is particularlysuitable for injection stretch blow molding applications. In particular,it provides a broad processing window, good process stability to preparecontainers with good thickness repartition, good surface aspects, goodfinishing, high ESCR and a high top load.

The injection-stretch blow molding process of the present invention caneither be a one-stage or a two-stage process. In a one-stage processinjection molding of the preform and blowing of the preform to the finaldesired shape are performed on the same machine, whereas in a two-stageprocess injection-molding of the preform and blowing of the preform areconducted in different machines, which can be separated by a longdistance. Thus, the two-stage process additionally requires the coolingof the preform to ambient temperature and a subsequent reheating beforethe blowing step.

It has now been surprisingly found that under stretching and blowingconditions similar to those used for polyethylene terephthalate,containers with high rigidity, high ESCR and high impact resistance canbe obtained.

The polyethylene resins according to the invention, having such aspecific composition, molecular weight and density, can lead to a markedimprovement of the processing properties when the resin is used ininjection-stretched-blow-moulding, while conserving or improvingmechanical behaviour as compared to the same articles prepared withother resins.

The present invention also comprises the method for preparing preforms,the preforms so obtained, the use of said preforms for preparingcontainers, and the containers prepared from said preforms.

Polyethylene resin is generally not used ininjection-stretch-blow-moulding applications and theinjection-stretch-blow-moulding conditions are thus adapted accordingly.

The preform, which has an open and a closed end, is prepared byinjection molding. For the present invention the polyethylene resinaccording to the invention is fed to an extruder, plasticized andinjected under pressure into an injection mold through an opening,generally referred to as “gate”. The polyethylene resin is injected intothe injection mold at an injection temperature of at least 220° C.,preferably of at least 230° C. The injection temperature is at most 300°C., preferably at most 290° C. and most preferably at most 280° C. Thechoice of injection temperature depends upon the melt flow index of thepolyethylene resin. It is clear to the skilled person that a lower meltflow index requires a higher injection temperature and vice versa. Theinjection mold is filled at such a rate as to give a ratio of moldfiling rate (in cm³/s) over gate size (in mm) of 15 or less, preferablyof 10 or less. The preform is cooled inside the injection mold andremoved from it. The ratio of mold filling rate over gate size variesdepending upon the viscosity of the molten polyethylene resin, i.e. amore viscous molten polyethylene resin requires a lower value for theratio than a more fluid molten polyethylene resin, so that a preformwith good processing properties in the subsequent stretch-blowing stepswill be obtained.

The two-step process comprises the steps of:

-   -   providing a preform by injection moulding on a mould, preferably        on a multi-cavity mould;    -   cooling the preform to room temperature;    -   transporting the preform to the blow moulding machine;    -   reheating the preform in the blow moulding machine in a        reflective radiant heat oven    -   optionally, passing the heated preform through an equilibration        zone to allow the heat to disperse evenly through the preform        wall;    -   optionally, submitting the preform to a pre-blow step;    -   stretching the preform axially by a centre rod;    -   orienting the stretched preform radially by high pressure air.

The one-step process comprises the steps of:

-   -   providing a pre-form by injection moulding on a mould,        preferably on a multi-cavity mould;    -   optionally slightly re-heating the pre-form;    -   optionally, passing the heated pre-form through an equilibration        zone to allow the heat to disperse evenly through the pre-form        wall;    -   optionally, submitting the preform to a pre-blow step;    -   stretching the pre-form axially by a centre rod;    -   orienting the stretched pre-form radially by high pressure air.

In a one-stage process the preform is cooled to a temperature in therange from 90° C. to 140° C. and is stretch-blown into a container. Allof these steps are performed on a single machine.

In a two-stage process the preform is allowed to cool to ambienttemperature and transported to a different machine. The preforms areuniformly reheated to a temperature below the polyethylene's meltingpoint. The reheating can be followed by an equilibration step.Subsequently, the preform is transferred to the stretch-blowing zone andsecured within the blowing mold, which has the same shape as the finalcontainer, in such a way that the closed end of the preform points tothe inside of the blowing mold. The preform is stretched axially with acenter rod, generally referred to as “stretch rod” to bring the wall ofthe perform against the inside wall of the blowing mold. The stretch rodspeed can go up to 2000 mm/s. Preferably it is in the range from 100mm/s to 2000 mm/s, and more preferably in the range from 500 mm/s to1500 mm/s. Pressurized gas is used to radially blow the preform into theblowing mold shape. The blowing is done using gas with a pressure in therange from 5 bars to 40 bars, and preferably from 10 bars to 30 bars.

The blowing of the preform can also be performed in two steps, by firstpre-blowing the preform with a lower gas pressure, and then blowing thepreform to its final shape with a higher gas pressure. The gas pressurein the pre-blowing step is in the range from 2 bars to 10 bars,preferably in the range from 4 bars to 6 bars. The preform is blown intoits final shape using gas with a pressure in the range from 5 bars to 40bars, more preferably from 10 bars to 30 bars, and most preferably from15 bars to 25 bars.

Following the stretching and blowing, the container is rapidly cooledand removed from the blowing mold.

The containers obtained by the injection-stretch blow molding process ofthe present invention are characterized by good impact properties incombination with high rigidity and high ESCR.

The articles prepared according to the present invention are hollowcontainers and bottles that can be used in various food and non-foodapplications, in particular for consumer packaging. The foodapplications comprise in particular the storage of juices, dry productsand dairy products. The non-food applications comprise in particular thestorage of cosmetic, detergents and pharmaceutical products.

EXAMPLES Example 1 1. Pellet Properties

The polyethylene resins of Grades X and Y have a bimodal molecularweight distribution produced in two serially connected slurry loopreactors i.e. a double loop reactor using a Ziegler-Natta catalystsystem and thus comprises two polyethylene fractions. The GPC are shownin FIG. 1. Grade Z is a polyethylene resin produced in the presence of ametallocene catalyst. Grades X and Z are comparative examples.

Table 1 shows the properties of the resins Grades X, Y and Z.

TABLE 1 Grade X Y Z DENSITY (kg/m³) 958 959 958 MI-2 (g/10 min) 0.3 0.877.8 HLMI (g/10 min) 30.1 77.6 173.5 ESCR 100% Igepal Co-630 >400 >400 22F50 (h) GPC Mn (g/mol) 11904 11033 19363 Mw (g/mol) 162481 126163 54548Mz (g/mol) 951002 846796 101876 d (Mw/Mn) 13.6 11.4 2.8 d′ (Mz/Mw) 5.96.7 1.9 Swell (%) Log shear rate 7.07 39 34.75 N/A 14.48 41.25 39.75 N/A28.8 46 46.75 N/A 71.5 56 54 N/A 142.5 63.75 67.5 N/A 272.1 75.25 77.75N/A 715.6 93.75 94 N/A N/A = Not applicable. It was impossible tomeasure the swell.

The density was measured according to the method of standard test ASTM1505 at a temperature of 23° C. The melt index MI2 and high load meltindex HLMI were measured by the method of standard test ASTM D 1238respectively under a load of 2.16 kg and 21.6 kg and at a temperature of190° C.

ESCR was measured according to ASTM D 1693 using 100% Igepal CO-630 as achemical agent.

The molecular weight distributions (MWD) d and d′ are defined by theratio Mw/Mn and Mz/Mw respectively where Mn (number average molecularweight), Mw (weight average molecular weight) and Mz (z-averagemolecular weight) are determined by gel permeation chromatography (GPC).MWD was measured as Mw/Mn (weight average molecular weight/numberaverage molecular weight) determined by GPC analysis.

The swell is measured on a Gottfert 2002 capillary rheometer accordingto ISO11443:2005 with the proviso that the extruded samples were 10 cmlong instead of 5 cm long. The method involves measuring the diameter ofthe extruded product at different shear velocities. The capillaryselection corresponds to a die having an effective length of 10 mm, adiameter of 2 mm and an aperture of 180°. The temperature is 210° C.Shear velocities range from 7 to 715 s⁻¹, selected in decreasing orderin order to reduce the time spent in the cylinder; 7 velocities areusually tested. When the extruded product has a length of about 10 cm,it is cut, after the pressure has been stabilised and the next velocityis selected. The extruded product (sample) is allowed to cool down in arectilinear position.

The diameter of the extruded product is then measured with an accuracyof 0.01 mm using a vernier, at 2.5 cm (d_(2.5)) and at 5 cm (d₅) fromone end of the sample, making at each position d_(2.5) and d₅ twomeasurements separated by an angle of 90°,

The diameter d_(o) the one end of the sample selected for the test isextrapolated:d _(o) =d _(2.5)+(d _(2.5) −d ₅)

The swell G is determined asG=100×(d _(o) −d _(f))/d _(f)wherein d_(f) is the die diameter.

The swell value is measured for each of the selected shear velocitiesand a graph representing the swell as a function of shear velocity canbe obtained.

With Grade Z which also has a higher melt flow, it was impossible toobtain an acceptable injection stretch blow moulded container, see FIG.2.

2. Injection Process

A preform (22 g) was injected with each of Grades X and Y as describedin Example 1, Table 1, and a standard commercial polyethyleneterephthalate (PET) on Arburg mono cavity machine.

The conditions used for the injections are given in Table 2.

TABLE 2 Conditions for injection Temperature (° C.) 220 Flow rate(cm3/s) 10 Injection speed (s) 1.4 Pressure (bar) 400

These conditions are the ones which provide the best preforms.

In Table 3, the surface aspects of the preforms are shown.

TABLE 3 Preforms Polyethylene terephthalate Grade X Grade Y (PET)Preforms Random flow No significant No marks lines (see FIG. 3) marks

Thus, it was observed that Grade B provides better, improved preformsover Grade A.

After this, these preforms were transformed into bottles by stretchingand blowing.

3. Stretching/Blowing Process

Bottles of 1 Liter were blown on a SIDEL SBO8 series 2. All tests wererealized with industrial equipments and industrial conditions (1700b/h). The heating was realized using a standard heating process asconventionally used for PET. The pressure during blowing was at 15 bar.

From the preform and bottle designs, the length ratio (3.09) and hoopratio (2.75) can be calculated.

The results on bottles obtained are given in the Table 4.

TABLE 4 Bottles' Properties Grade Grade X Grade Y PET Surface aspect/ ++++ +++ finishing (see (see FIG. 4) FIG. 5) Molded drawings + +++ ++Bottle weight g 22 23 21 Thickness repartition mm 0.2324 0.2509 0.1342(horizontal) variability 24% 12%  8% Thickness repartition mm 0.28130.2746 0.1696 (vertical) variability 58% 39% 59% Dynamical Fmax 71 69 61compression (about (ISO 12048) for 4 mm) Drop Impact Resistance F50 -m >6 >6 5.9 (Drop Test: 1 L water at room T ° C.) Molded drawings =quality of engravings

The drop tests were carried out with bottles filled with 1 liter ofwater at room temperature. The bottles were then dropped from increasingheight, until 50% of the bottles dropped were cracked.

Grade Y shows improved aspects in comparison with predecessor Grade Xi.e.

-   -   better surface aspect and finishing    -   better moulded drawings/quality of engravings    -   less thickness variability both vertically and horizontally    -   whilst maintaining an equally good drop impact resistance and        dynamical compression

We show here that Grade Y according to the invention has propertiescomparable to PET. Moreover the moulded drawings (engravings) are muchmore accurate with Grade Y according to the invention than when usingPET.

Example 2

Furthermore, FIGS. 6 and 7 show bottle schematics and a full view of anISBM bottle prepared with the resin according to the invention i.e.Grade Y. It was observed that even mouldings with dimensionalrestrictions i.e. narrower portions, can be successfully made using theresin of the invention. Furthermore, it was observed that bottles of 100dm³ with a weight of only 22 g could be obtained, whilst maintaining allother properties. Thus the resin according to the invention enablesoverall reduction in weight without deteriorating other properties of anISBM bottle.

The invention claimed is:
 1. A polyethylene resin having a multimodalmolecular weight distribution comprising at least two polyethylenefractions A and B, fraction A being substantially free of comonomer andhaving a lower weight average molecular weight and a higher density thanfraction B, each fraction prepared in different reactors of two reactorsconnected in series in the presence of a Ziegler-Natta catalyst system,the polyethylene resin having a density of from 0.950 to 0.965 g/cm³,measured following the method of standard test ASTM 1505 at atemperature of 23° C., a melt index MI2 of from 0.5 to 5 g/10 min,measured following the method of standard test ASTM D 1238 at atemperature of 190° C. and under a load of 2.16 kg, and molecular weightdistribution Mw/Mn of from 5 to 20, wherein the fraction B has a highload melt index HL275 of at least 1.5 dg/min and at most 14 dg/min. 2.The polyethylene resin of claim 1, wherein the Ziegler-Natta catalystsystem comprises a Ziegler-Natta catalyst component D and apreactivating agent, wherein the Ziegler Natta catalyst component D isobtained by a) generating a reaction product A by contacting a magnesiumdialkoxide compound with a halogenating agent; b) contacting reactionproduct A with a first halogenating/titanating agent to form reactionproduct B; c) contacting reaction product B with a secondhalogenating/titanating agent to form reaction product C; and d)contacting reaction product C with a third halogenating/titanating agentto form catalyst component D.
 3. The polyethylene resin according toclaim 2, wherein the preactivating agent of the Ziegler-Natta catalystsystem is an organoaluminium compound.
 4. The polyethylene resinaccording to claim 1, wherein at least one of the reactors is a slurryloop reactor.
 5. The polyethylene resin according to claim 4, whereinthe two reactors are slurry loop reactors.
 6. The polyethylene resinaccording to claim 1, wherein fraction B is produced in a first reactorof the two reactors and fraction A is produced in a second reactor ofthe two reactors.
 7. The polyethylene resin according to claim 1,wherein the polyethylene resin has an environmental stress crackresistance of at least 100 h.
 8. A process comprising injection stretchblow moulding a polyethylene resin having a multimodal molecular weightdistribution comprising at least two polyethylene fractions A and B,fraction A being substantially free of comonomer and having a lowerweight average molecular weight and a higher density than fraction B,each fraction prepared in different reactors of two reactors connectedin series in the presence of a Ziegler-Natta catalyst system, thepolyethylene resin having a density of from 0.950 to 0.965 g/cm³,measured following the method of standard test ASTM 1505 at atemperature of 23° C., a melt index MI2 of from 0.5 to 5 g/10 min,measured following the method of standard test ASTM D 1238 at atemperature of 190° C. and under a load of 2.16 kg, and molecular weightdistribution Mw/Mn of from 5 to
 20. 9. The polyethylene resin of claim1, wherein the Ziegler-Natta catalyst system comprises a titaniumcompound having at least one titanium-halogen bond.
 10. The polyethyleneresin of claim 1, wherein the polyethylene resin has a bimodal molecularweight distribution.
 11. The polyethylene resin of claim 1, wherein thefraction B has a density of at least 0.908 g/cm³ and at most 0.945g/cm³, measured following the method of standard test ASTM 1505 at atemperature of 23° C.
 12. The polyethylene resin of claim 1, wherein thefraction B has a high load melt index HL275 of at least 5 dg/min and atmost 10 dg/min.
 13. The polyethylene resin of claim 1, wherein thefraction A has a density of at least 0.953 g/cm³ and at most 0.978g/cm³, measured following the method of standard test ASTM 1505 at atemperature of 23° C.
 14. The polyethylene resin of claim 1, wherein thepolyethylene resin comprises from 36 to 50 wt % of the fraction B andfrom 50 to 64 wt % of the fraction A.
 15. The process of claim 8,wherein the Ziegler-Natta catalyst system comprises a titanium compoundhaving at least one titanium-halogen bond and an internal electrondonor, both on a support, and an organoaluminum compound.
 16. Theprocess of claim 8, wherein the fraction B has a high load melt indexHL275 of at least 1.5 dg/min and at most 14 dg/min.